High temperature resistant sheet materials well-known to the art and are used under various high temperature conditions. These sheet materials are known in the art as "papers", since they are often made by method similar to paper-making methods, although the thickness thereof can be up to one half inch or more. These papers are used in a variety of high temperature environments such as protective linings, high temperature insulators, and filter media for high temperature fluids. Most often, these papers are made by laying fibers, particularly inorganic fibers such as ceramic or glass fibers, into a matt and consolidating the matt into a paper, although other processes may be used. The fibers used in making such papers, by virtue of the process of laying the fibers, are randomly oriented and, with consolidation, are interlocked together into the form of a shape sustaining paper having two lateral surfaces. Such a consolidated paper has considerable structural integrity due to the random orientation and interlocking of the fibers, and such structural integrity is quite sufficient for many uses of the papers. However, that inherent structural integrity of the papers is not sufficient for other applications where the papers are subjected to higher stresses, e.g., where the papers are subjected to mechanical action such as abrasion and where the papers are subjected to higher pressures such as in fluid filtration. Under these higher stress environments, the structural integrity of the papers is quickly deteriorated by action of the stresses on the papers and the papers will quickly become unserviceable.
To avoid the loss of structural integrity of the papers when placed in a higher stress environment, these papers often have a binder applied thereto. The binders take various forms, but generally, the binders are organic polymers such as phenolics, acrylics and epoxies. The binders serve to improve the structural integrity of the papers during manufacture and fabrication of the papers into products and maintain that structural integrity in higher stress environments.
However, the binders of these papers, while quite satisfactory for ambient or slightly elevated temperatures, will begin to lose the binding effect at higher temperatures, e.g., about 300.degree.-400.degree. F., with a concomitant loss of structural integrity of the papers. With continued use at these temperatures, the binder will burn away and the structural integrity of the papers will again depend entirely upon the interlocking of the fibers. When this occurs, and when the papers are subjected to stress environments, the papers will quickly fail, e.g., by rupturing, tearing and the like.
In U.S. Pat. No. 4,499,134, the entire disclosure of which is incorporated herein by reference, and a patent of the present assignee, a composite is described which is made of a combination of the papers and an abrasion resistant, high temperature resistant, flexible woven or non-woven scrim. The paper and the scrim are stitched together by a network of abrasion resistant high temperature threads in a repeating pattern. With that composite, even though the binder in the paper burns away in use, the scrim provides structural integrity to the composite and prevents deterioration of the paper when subjected to mechanical action, e.g., abrasion such as encountered in a rotary kiln where the composite is between the kiln wall and the fire bricks. While this composite represents a very substantial improvement in the art, especially in uses where the composite is subjected to mechanical abrasion, it has been found that the composite of that patent is not necessarily acceptable for other high stress environments such as filtration of high temperature fluid streams, and especially where that filtration must be to a very high order of filtration efficiency.
In this latter regard, among other reasons, the needles used for stitching the paper and the scrim together introduce holes through the composite. While the number of these holes is relatively small compared with the total surface area of the composite, nevertheless, these holes do represent a discontinuity in the filtering efficiency of the composite. Such discontinuity can be of substantial significance when the filter is intended for use as a high efficiency filter, as opposed to other general purpose filters such as those which would be used in conventional baghouse filters and the like.
It would be, therefore, of considerable advantage to the art to provide such composites, of the nature described in U.S. Pat. No. 4,499,134, for use at higher temperatures where the binder burns away with use, but wherein the structural integrity of the papers can be largely maintained at the higher temperatures and under the stress conditions without the necessity of stitching the scrim to the papers and without the discontinuities of filter efficiencies.